Mass spectrometers include instruments which ionize a gas sample, separate a resulting beam of ions by mass to charge ratio, and then detect filtered ions as an electrical signal. The masses, unique for each substance, identifies the gas molecules from which the ions were created. One such mass filter used is referred to a quadrupole which consists of four parallel electrodes or poles arranged in a square array. Opposite poles are connected together electrically such that an electric field of hyperbolic geometry is produced. Potentials applied to these poles are a superposition of variable DC and RF voltages, generally of a fixed RF frequency.
The above instrument works preferably in a high vacuum environment because the ions, once created, must not collide with other gas molecules as the molecules move through the instrument; otherwise some molecules may not be detected. High vacuum means pressures below 1.3E-2 Pascals, or greater, or approximately 1-0E-4 torr.
It has been determined that the transmission of ions in a quadrupole mass spectrometer suffers losses in the high operating pressure area due to several effects; most notably from collisions between ions and neutral gas molecules, ion scattering and coulombic repulsion. The above effects results in a perceived non-linearity between pressure and ion current. It would be desirable to provide a sensor which could correct such non-linear deficiencies, or alternately to provide a technique for correcting measured ion current data to expand the performance of a quadrupole mass analyzer in a higher pressure regime than currently available.